Ligands Having Extended tt Systems

The simplest case of an organic molecule having a linear tt system is ethylene, which has a single tt bond resulting from the interactions of two 2p orbitals on its carbon atoms. Interactions of these p orbitals result in one bonding and one antibonding tt orbital, as shown  

The antibonding interaction has a nodal plane perpendicular to the intemuclear axis, but the bonding interaction has no such nodal plane. 

Next is the three-atom tt system, the Tr-allyl radical, C3H5. In this case, there are three 2p orbitals to be considered, one from each of the carbon atoms participating in the tt system. The possible interactions are as follows  

The number of nodes perpendicular to the carbon chain increases in going from lower energy to higher energy orbitals for example, in the ir-allyl system, the number of nodes increases from zero to one to two from the lowest to the highest energy orbital. This is a trend that will also appear in the following examples. 

One more example should suffice to illustrate this procedure. 1,3-Butadiene may exist in cis or trans forms. For our purposes, we will treat both as linear systems the nodal behavior of the molecular orbitals is the same in each case as in a linear tt system of four atoms. The 2p orbitals of the carbon atoms in the chain may interact in four ways, with the lowest energy tt molecular orbital having all constructive interactions between neighboring p orbitals, and the energy of the other tt orbitals increasing with the number of nodes between the atoms. 

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The electron counting method of choice is a matter of individual preference. Method A includes the formal oxidation state of the metal Method B does not. Method B may be simpler to use for ligands having extended tt systems for example, ligands have an electron count of 5, ligands an electron count of 3, and so on. Because neither description describes the bonding in any real sense, these methods should, like the Lewis electron-dot approach in main group chemistry, be considered primarily electron... 

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